/*
 * Copyright 2012 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.channel.nio;


import io.netty.channel.Channel;
import io.netty.channel.ChannelException;
import io.netty.channel.EventLoopException;
import io.netty.channel.SingleThreadEventLoop;
import io.netty.channel.nio.AbstractNioChannel.NioUnsafe;
import io.netty.util.concurrent.TaskScheduler;
import io.netty.util.internal.SystemPropertyUtil;
import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.io.IOException;
import java.nio.channels.CancelledKeyException;
import java.nio.channels.SelectableChannel;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.spi.SelectorProvider;
import java.util.ArrayList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.Queue;
import java.util.Set;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * {@link SingleThreadEventLoop} implementation which register the {@link Channel}'s to a
 * {@link Selector} and so does the multi-plexing of these in the event loop.
 *
 */
public final class NioEventLoop extends SingleThreadEventLoop {

    /**
     * Internal Netty logger.
     */
    private static final InternalLogger logger =
            InternalLoggerFactory.getInstance(NioEventLoop.class);

    private static final int CLEANUP_INTERVAL = 256; // XXX Hard-coded value, but won't need customization.

    private static final boolean EPOLL_BUG_WORKAROUND =
            SystemPropertyUtil.getBoolean("io.netty.epollBugWorkaround", false);

    private static final long SELECT_TIMEOUT = SystemPropertyUtil.getLong("io.netty.selectTimeout", 500);
    private static final long SELECT_TIMEOUT_NANOS = TimeUnit.MILLISECONDS.toNanos(SELECT_TIMEOUT);

    // Workaround for JDK NIO bug.
    //
    // See:
    // - http://bugs.sun.com/view_bug.do?bug_id=6427854
    // - https://github.com/netty/netty/issues/203
    static {
        String key = "sun.nio.ch.bugLevel";
        try {
            String buglevel = System.getProperty(key);
            if (buglevel == null) {
                System.setProperty(key, "");
            }
        } catch (SecurityException e) {
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to get/set System Property '" + key + '\'', e);
            }
        }
        if (logger.isDebugEnabled()) {
            logger.debug("Using select timeout of " + SELECT_TIMEOUT);
            logger.debug("Epoll-bug workaround enabled = " + EPOLL_BUG_WORKAROUND);
        }
    }
    /**
     * The NIO {@link Selector}.
     */
    Selector selector;

    private final SelectorProvider provider;

    /**
     * Boolean that controls determines if a blocked Selector.select should
     * break out of its selection process. In our case we use a timeone for
     * the select method and the select method will block for that time unless
     * waken up.
     */
    private final AtomicBoolean wakenUp = new AtomicBoolean();

    private volatile int ioRatio = 50;
    private int cancelledKeys;
    private boolean cleanedCancelledKeys;

    NioEventLoop(
            NioEventLoopGroup parent, ThreadFactory threadFactory,
            TaskScheduler scheduler, SelectorProvider selectorProvider) {
        super(parent, threadFactory, scheduler);
        if (selectorProvider == null) {
            throw new NullPointerException("selectorProvider");
        }
        provider = selectorProvider;
        selector = openSelector();
    }

    private Selector openSelector() {
        try {
            return provider.openSelector();
        } catch (IOException e) {
            throw new ChannelException("failed to open a new selector", e);
        }
    }

    @Override
    protected Queue<Runnable> newTaskQueue() {
        // This event loop never calls takeTask()
        return new ConcurrentLinkedQueue<Runnable>();
    }

    /**
     * Registers an arbitrary {@link SelectableChannel}, not necessarily created by Netty, to the {@link Selector}
     * of this event loop.  Once the specified {@link SelectableChannel} is registered, the specified {@code task} will
     * be executed by this event loop when the {@link SelectableChannel} is ready.
     */
    public void register(final SelectableChannel ch, final int interestOps, final NioTask<?> task) {
        if (ch == null) {
            throw new NullPointerException("ch");
        }
        if (interestOps == 0) {
            throw new IllegalArgumentException("interestOps must be non-zero.");
        }
        if ((interestOps & ~ch.validOps()) != 0) {
            throw new IllegalArgumentException(
                    "invalid interestOps: " + interestOps + "(validOps: " + ch.validOps() + ')');
        }
        if (task == null) {
            throw new NullPointerException("task");
        }

        if (isShutdown()) {
            throw new IllegalStateException("event loop shut down");
        }

        try {
            ch.register(selector, interestOps, task);
        } catch (Exception e) {
            throw new EventLoopException("failed to register a channel", e);
        }
    }

    void executeWhenWritable(AbstractNioChannel channel, NioTask<SelectableChannel> task) {
        if (channel == null) {
            throw new NullPointerException("channel");
        }

        if (isShutdown()) {
            throw new IllegalStateException("event loop shut down");
        }

        SelectionKey key = channel.selectionKey();
        channel.writableTasks.offer(task);

        int interestOps = key.interestOps();
        if ((interestOps & SelectionKey.OP_WRITE) == 0) {
            key.interestOps(interestOps | SelectionKey.OP_WRITE);
        }
    }

    /**
     * Returns the percentage of the desired amount of time spent for I/O in the event loop.
     */
    public int getIoRatio() {
        return ioRatio;
    }

    /**
     * Sets the percentage of the desired amount of time spent for I/O in the event loop.  The default value is
     * {@code 50}, which means the event loop will try to spend the same amount of time for I/O as for non-I/O tasks.
     */
    public void setIoRatio(int ioRatio) {
        if (ioRatio <= 0 || ioRatio >= 100) {
            throw new IllegalArgumentException("ioRatio: " + ioRatio + " (expected: 0 < ioRatio < 100)");
        }
        this.ioRatio = ioRatio;
    }

    /**
     * Replaces the current {@link Selector} of this event loop with newly created {@link Selector}s to work
     * around the infamous epoll 100% CPU bug.
     */
    public void rebuildSelector() {
        if (!inEventLoop()) {
            execute(new Runnable() {
                @Override
                public void run() {
                    rebuildSelector();
                }
            });
            return;
        }

        final Selector oldSelector = selector;
        final Selector newSelector;

        if (oldSelector == null) {
            return;
        }

        try {
            newSelector = Selector.open();
        } catch (Exception e) {
            logger.warn("Failed to create a new Selector.", e);
            return;
        }

        // Register all channels to the new Selector.
        int nChannels = 0;
        for (;;) {
            try {
                for (SelectionKey key: oldSelector.keys()) {
                    Object a = key.attachment();
                    try {
                        if (key.channel().keyFor(newSelector) != null) {
                            continue;
                        }

                        int interestOps = key.interestOps();
                        key.cancel();
                        key.channel().register(newSelector, interestOps, a);
                        nChannels ++;
                    } catch (Exception e) {
                        logger.warn("Failed to re-register a Channel to the new Selector.", e);
                        if (a instanceof AbstractNioChannel) {
                            AbstractNioChannel ch = (AbstractNioChannel) a;
                            ch.unsafe().close(ch.unsafe().voidFuture());
                        } else {
                            @SuppressWarnings("unchecked")
                            NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                            invokeChannelUnregistered(task, key, e);
                        }
                    }
                }
            } catch (ConcurrentModificationException e) {
                // Probably due to concurrent modification of the key set.
                continue;
            }

            break;
        }

        selector = newSelector;

        try {
            // time to close the old selector as everything else is registered to the new one
            oldSelector.close();
        } catch (Throwable t) {
            if (logger.isWarnEnabled()) {
                logger.warn("Failed to close the old Selector.", t);
            }
        }

        logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
    }

    @Override
    protected void run() {
        // use 80% of the timeout for measure
        final long minSelectTimeout = SELECT_TIMEOUT_NANOS / 100 * 80;

        Selector selector = this.selector;
        int selectReturnsImmediately = 0;

        for (;;) {
            wakenUp.set(false);

            try {
                if (hasTasks()) {
                    selectNow();
                } else {
                    long beforeSelect = System.nanoTime();
                    int selected = select();
                    if (EPOLL_BUG_WORKAROUND) {
                        if (selected == 0) {
                            long timeBlocked = System.nanoTime()  - beforeSelect;
                            if (timeBlocked < minSelectTimeout) {
                                // returned before the minSelectTimeout elapsed with nothing select.
                                // this may be the cause of the jdk epoll(..) bug, so increment the counter
                                // which we use later to see if its really the jdk bug.
                                selectReturnsImmediately ++;
                            } else {
                                selectReturnsImmediately = 0;
                            }
                            if (selectReturnsImmediately == 10) {
                                // The selector returned immediately for 10 times in a row,
                                // so recreate one selector as it seems like we hit the
                                // famous epoll(..) jdk bug.
                                rebuildSelector();
                                selector = this.selector;
                                selectReturnsImmediately = 0;

                                // try to select again
                                continue;
                            }
                        } else {
                            // reset counter
                            selectReturnsImmediately = 0;
                        }
                    }

                    // 'wakenUp.compareAndSet(false, true)' is always evaluated
                    // before calling 'selector.wakeup()' to reduce the wake-up
                    // overhead. (Selector.wakeup() is an expensive operation.)
                    //
                    // However, there is a race condition in this approach.
                    // The race condition is triggered when 'wakenUp' is set to
                    // true too early.
                    //
                    // 'wakenUp' is set to true too early if:
                    // 1) Selector is waken up between 'wakenUp.set(false)' and
                    //    'selector.select(...)'. (BAD)
                    // 2) Selector is waken up between 'selector.select(...)' and
                    //    'if (wakenUp.get()) { ... }'. (OK)
                    //
                    // In the first case, 'wakenUp' is set to true and the
                    // following 'selector.select(...)' will wake up immediately.
                    // Until 'wakenUp' is set to false again in the next round,
                    // 'wakenUp.compareAndSet(false, true)' will fail, and therefore
                    // any attempt to wake up the Selector will fail, too, causing
                    // the following 'selector.select(...)' call to block
                    // unnecessarily.
                    //
                    // To fix this problem, we wake up the selector again if wakenUp
                    // is true immediately after selector.select(...).
                    // It is inefficient in that it wakes up the selector for both
                    // the first case (BAD - wake-up required) and the second case
                    // (OK - no wake-up required).

                    if (wakenUp.get()) {
                        selector.wakeup();
                    }
                }

                cancelledKeys = 0;

                final long ioStartTime = System.nanoTime();
                processSelectedKeys();
                selector = this.selector;
                final long ioTime = System.nanoTime() - ioStartTime;

                final int ioRatio = this.ioRatio;
                runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                selector = this.selector;

                if (isShutdown()) {
                    closeAll();
                    if (confirmShutdown()) {
                        break;
                    }
                }
            } catch (Throwable t) {
                logger.warn("Unexpected exception in the selector loop.", t);

                // Prevent possible consecutive immediate failures that lead to
                // excessive CPU consumption.
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // Ignore.
                }
            }
        }
    }

    @Override
    protected void cleanup() {
        try {
            selector.close();
        } catch (IOException e) {
            logger.warn("Failed to close a selector.", e);
        }
    }

    void cancel(SelectionKey key) {
        key.cancel();
        cancelledKeys ++;
        if (cancelledKeys >= CLEANUP_INTERVAL) {
            cancelledKeys = 0;
            cleanedCancelledKeys = true;
            cleanupKeys();
        }
    }

    private void processSelectedKeys() {
        Set<SelectionKey> selectedKeys = selector.selectedKeys();
        // check if the set is empty and if so just return to not create garbage by
        // creating a new Iterator every time even if there is nothing to process.
        // See https://github.com/netty/netty/issues/597
        if (selectedKeys.isEmpty()) {
            return;
        }

        Iterator<SelectionKey> i;
        cleanedCancelledKeys = false;
        boolean clearSelectedKeys = true;
        try {
            for (i = selectedKeys.iterator(); i.hasNext();) {
                final SelectionKey k = i.next();
                final Object a = k.attachment();
                if (a instanceof AbstractNioChannel) {
                    processSelectedKey(k, (AbstractNioChannel) a);
                } else {
                    @SuppressWarnings("unchecked")
                    NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                    processSelectedKey(k, task);
                }

                if (cleanedCancelledKeys) {
                    // Create the iterator again to avoid ConcurrentModificationException
                    if (selectedKeys.isEmpty()) {
                        clearSelectedKeys = false;
                        break;
                    } else {
                        i = selectedKeys.iterator();
                    }
                }
            }
        } finally {
            if (clearSelectedKeys) {
                selectedKeys.clear();
            }
        }
    }

    private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
        final NioUnsafe unsafe = ch.unsafe();
        if (!k.isValid()) {
            // close the channel if the key is not valid anymore
            unsafe.close(unsafe.voidFuture());
            return;
        }

        int readyOps = -1;
        try {
            readyOps = k.readyOps();
            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                unsafe.read();
                if (!ch.isOpen()) {
                    // Connection already closed - no need to handle write.
                    return;
                }
            }
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                processWritable(k, ch);
            }
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                // See https://github.com/netty/netty/issues/924
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);

                unsafe.finishConnect();
            }
        } catch (CancelledKeyException e) {
            if (readyOps != -1 && (readyOps & SelectionKey.OP_WRITE) != 0) {
                unregisterWritableTasks(ch);
            }
            unsafe.close(unsafe.voidFuture());
        }
    }

    private static void processWritable(SelectionKey k, AbstractNioChannel ch) {
        if (ch.writableTasks.isEmpty()) {
            ch.unsafe().flushNow();
        } else {
            NioTask<SelectableChannel> task;
            for (;;) {
                task = ch.writableTasks.poll();
                if (task == null) { break; }
                processSelectedKey(ch.selectionKey(), task);
            }
            k.interestOps(k.interestOps() | SelectionKey.OP_WRITE);
        }
    }

    private static void unregisterWritableTasks(AbstractNioChannel ch) {
        NioTask<SelectableChannel> task;
        for (;;) {
            task = ch.writableTasks.poll();
            if (task == null) {
                break;
            } else {
                invokeChannelUnregistered(task, ch.selectionKey(), null);
            }
        }
    }

    private static void processSelectedKey(SelectionKey k, NioTask<SelectableChannel> task) {
        int state = 0;
        try {
            task.channelReady(k.channel(), k);
            state = 1;
        } catch (Exception e) {
            k.cancel();
            invokeChannelUnregistered(task, k, e);
            state = 2;
        } finally {
            switch (state) {
            case 0:
                k.cancel();
                invokeChannelUnregistered(task, k, null);
                break;
            case 1:
                if (!k.isValid()) { // Cancelled by channelReady()
                    invokeChannelUnregistered(task, k, null);
                }
                break;
            }
        }
    }

    private void closeAll() {
        cleanupKeys();
        Set<SelectionKey> keys = selector.keys();
        Collection<AbstractNioChannel> channels = new ArrayList<AbstractNioChannel>(keys.size());
        for (SelectionKey k: keys) {
            Object a = k.attachment();
            if (a instanceof AbstractNioChannel) {
                channels.add((AbstractNioChannel) a);
            } else {
                k.cancel();
                @SuppressWarnings("unchecked")
                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                invokeChannelUnregistered(task, k, null);
            }
        }

        for (AbstractNioChannel ch: channels) {
            unregisterWritableTasks(ch);
            ch.unsafe().close(ch.unsafe().voidFuture());
        }
    }

    private static void invokeChannelUnregistered(NioTask<SelectableChannel> task, SelectionKey k, Throwable cause) {
        try {
            task.channelUnregistered(k.channel(), cause);
        } catch (Exception e) {
            logger.warn("Unexpected exception while running NioTask.channelUnregistered()", e);
        }
    }

    @Override
    protected void wakeup(boolean inEventLoop) {
        if (!inEventLoop && wakenUp.compareAndSet(false, true)) {
            selector.wakeup();
        }
    }

    void selectNow() throws IOException {
        try {
            selector.selectNow();
        } finally {
            // restore wakup state if needed
            if (wakenUp.get()) {
                selector.wakeup();
            }
        }
    }

    private int select() throws IOException {
        try {
            return selector.select();
        } catch (CancelledKeyException e) {
            if (logger.isDebugEnabled()) {
                logger.debug(
                        CancelledKeyException.class.getSimpleName() +
                                " raised by a Selector - JDK bug?", e);
            }
            // Harmless exception - log anyway
        }

        return -1;
    }

    void cleanupKeys() {
        try {
            selector.selectNow();
        } catch (Throwable t) {
            logger.warn("Failed to update SelectionKeys.", t);
        }
    }
}